This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The molecular basis for alloreactive T cells and their specificity/degeneracy for pMHC ligands remain unresolved. Using a large scale screening approach involving 60 novel I-Ek alloreactive T cells and 83 naturally processed self-peptides, we identified nine T cells for which there were matching stimulatory self-peptides. Six of these T cells recognized single peptides with the same specificity observed for conventional recognition. Three of the T cells were stimulated by two or three distinct peptides, which had no sequence homology. These multiply reactive T cells recognized each pMHC specifically, but utilized distinct constellations of I-Ek contact residues. These studies reveal that allorecognition is highly specific, and that each T cell has an inherent capacity to recognize multiple distinct ligands, providing an explanation for the high frequency of alloreactivity.